1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to the use of a lithium/fluorinated carbon, for example, a Li/CFx cell, as a power source for an implantable medical device. Such applications require the cell to discharge under a light load for extended periods of time interrupted from time-to-time by pulse discharge. Specifically, the cell is ideal for use in an implantable cardioverter defibrillator.
2. Prior Art
Currently, implantable cardioverter defibrillators (ICD's) are powered by lithium/silver vanadium oxide (Li/SVO) cells. This chemistry provides excellent high-rate pulsing capability. Despite their high power, however, Li/SVO cells demonstrate time-dependent resistance growth during middle-of-life under some usage conditions. This resistance growth can reduce the ability of the cell to deliver energy as quickly as desired. Ideally, ICD's require a cell chemistry whose performance is easily predictable and not time-dependent.
An exemplary chemistry that meets these requirements is of a lithium/carbon monofluoride couple (Li/CFx). This system is currently used to power implantable medical devices with intermediate power requirements. Despite its excellent stability, however, the Li/CFx system has not been used for demanding applications such as ICD's because it is not believed to have sufficient power capability. Existing Li/CFx cell technology has a maximum power capability of 0.05 W/cc. The limited power capability is a direct result of low electrolyte conductivity (<1×10−2 S/cm at 37° C.), high electrical resistivity of the cathode matrix (>50 ohm*cm), and a low anode-to-cathode interface area of 5 cm2 to 20 cm2.